Measurements on loudspeakers are made by various sorts of individuals and for a variety of reasons. At Amplify Labs, we conduct loudspeaker measurements for a variety of purposes. Our measurements range from assessing T/S parameters at the component level to evaluating complete speaker enclosures, ensuring they meet the published specifications. We also perform in-situ measurements on complete devices to evaluate their performance in realistic usage scenarios.
To be truly valuable, these measurements must be accurate, meaningful, and repeatable. Accuracy is achieved through the use of high-quality test equipment and standardized procedures that prioritize reliability and error prevention. Meaningfulness is attained when measurements correlate as closely as possible with subjective experiences. Repeatability is ensured by adhering to strict rules regarding instrumentation, placement of test microphones, and the specified acoustic environment. At Amplify Labs, we prioritize these goals throughout the design and execution of our acoustic measurements.
We recently focused on developing a lightweight IEC baffle to enhance the accuracy and repeatability of our measurements. This innovative solution has shown promising results, and we are excited to document and share the entire process with you.
Why do we need an IEC baffle
When acoustic waves, traveling in air, meet a sharp edge, edge diffraction occurs. Diffracted waves will travel with direct waves from the speaker and skew the loudspeaker measurement. As we can see in Fig 1, as the direct wave from the source travel across the speaker enclosure and hit the corners, secondary sources are formed and start to radiate sound as well. The samples and devices we receive here at Amplify Labs come in all shapes and sizes. If not controlled, edge diffraction will inevitably impact the accuracy of our measurements.
Eliminate Baffle Step:
Baffle step is another problem related to testing loudspeakers of different sizes. Baffle step is created when the sound radiation from the loudspeaker contracts from 4pi space to 2pi space. The difference in radiating space will cause a 6 dB difference in relative sound pressure level (Fig 2). Different enclosure sizes will create different baffle step effects and make it impossible for us to compare different speakers fairly.
Fig 3 demonstrates an ideal piston mounted in different sized circular baffles. The on-axis frequency response can change drastically depending on the size of the baffle. Therefore, we need to create a large enough baffle to act like an infinite baffle in the frequency range of interest. This is especially important when measuring larger drivers that can play down to lower frequencies.
If you would like to learn more about baffle diffraction and sound radiation patterns, no one explains it better than Dr. Linkwitz. You can find more of his work on this topic here.
Standardized Baffle Size:
Standardization in testing is essential for making repeatable tests, not only here at our lab, but also across different labs in other parts of the world. The IEC 268-5 baffle standard is well established and used by many labs around the world. Having a baffle of the same size will make it easier to compare our results with results from other labs. Creating a testing procedure based on mounting the speaker under test inside the baffle will also improve the repeatability of our tests.
Before construction of our baffle, we asked other labs to send pictures of their baffle so we can gain a general understanding of what is being used in the field (Fig 4). After some research, we found that these baffles are usually constructed with thick and heavy MDF boards on large metal stands with some speaker mounting feature in the middle. The heavyweight nature of the MDF boards are beneficial for acoustic measurements, as the mass of the material will resist vibrations caused by the force exerted from the speaker under test. This can be explained by Newton’s Second Law (F=ma). Force exerted by the speaker can be seen as a constant, and the more mass there is on the baffle, the less acceleration there will be. This will translate to less unwanted movement on the baffle.
As much as we want to construct a similar baffle for our lab, we face a unique set of constraints that will not allow us to do so. Firstly, our lab is located in a basement, necessitating the transportation of all equipment through a small staircase. The difficulty of carrying larger and heavier objects makes it impractical for us to bring down a MDF baffle in one piece. In addition, we have a limited amount of test space so we have no intention to set up a heavy baffle in a permanent position. Ideally, we want a baffle that can be easily moved so we can set it up only when necessary. Therefore, we need to come up with an effective yet lightweight solution to satisfy our needs.
In our next blog post, we will demonstrate the creation process of this baffle, tests and improvements on the baffle material damping characteristics, as well as some speaker measurement data with and without the baffle.